Heating and cooling of internal spaces of residences and small commercial structures have not always been achieved at maximized fuel efficiency. It is well-known that there are a number of factors which contribute to the loss of fuel efficiency and waste of energy.
For example, in many forced air heating systems, with and without air conditioning, a substantial amount of energy is wasted heating the combustion chamber and surrounding furnace cabinet each time the heating cycle is initiated and combustion commenced. There must, of course, be an exhaust for combustion products and, therefore, a certain amount of heat energy is necessarily lost through the stack. However, only a portion of the losses that occur in many installations is necessary to facilitate exhaust of combustion products.
Various techniques have been attempted to reduce losses. Flue dampers, for example, raise safety concerns, since combustion exhaust products may not be properly exhausted if the flue damper operates improperly. The result could be dangerous if exhaust products enter the structure being heated. Flue dampers, of course, are designed to be open when the furnace is being operated and, therefore, are not designed to alter the operational characteristics of the furnace, but only reduce stack losses in the absence of combustion and when the furnace is not being operated.
The operational characteristics of the furnaces themselves also do not maximize energy efficiencies. Typical operational cycles of heating and cooling systems fail to fully consider what can be done to increase the efficiencies and reduce undesirable losses. A major source of heat loss occurs through what is commonly referred to as "stand-by losses". In forced air systems, many stand-by losses occur because of the nature of the operational cycle normally used. For example, when a thermostat senses that heat is required, a burner is operated to produce combustion in a combustion chamber. Heat is produced not only in the combustion chamber, but in a heat exchanger over which the forced air flows to carry heat from the furnace into the spaces to be heated. The air returns from these spaces to be reheated in the furnace.
In typical operation of forced air and many other furnaces, air circulation does not commence until the air adjacent the heat exchanger is heated to a selected temperature. As a result, stack temperatures and temperatures within the heat exchanger rise rapidly to a level far beyond that which is needed for combustion gas exhaust. It should be understood that some heating of the stack is required in order to produce the desired exhaust effect, as is well-known. However, when excess heat is produced prior to initiation of the forced air circulation, the excess heat is lost up the stack.
In standard furnace operation, a thermal sensor in the air plenum adjacent to and downstream of the heat exchanger is used to effect energization of the circulating fan to initiate air circulation when the temperature of the air in the air plenum reaches a pre-selected value. However, since the air is not circulated initially, it takes longer to heat the stationary air in the plenum than it would to heat air flowing across the heat exchanger. The efficiency of the heat exchanger, which is designed to transfer heat to moving air in contact therewith, is substantially reduced when the air is not circulating.
Thereafter, when the thermostat senses that the desired temperature in the space being heated has been achieved, the control circuit to operate the burner is deenergized and combustion is terminated. The fan continues to circulate air until the temperature sensor in the downstream air plenum senses a drop in the temperature of the air to a second pre-selected value below the value at which circulation was commenced, at which time the fan is shut down. In typical systems, the shut-down temperature differential may be 20.degree. below the temperature at which operation of the fan was commenced.
Even though air circulation is terminated as a result of the temperature of the circulating air
recognized reaching the shut-down value, it should be that at the typical shut-down temperature of 120.degree. F., there is still considerable heat left in the circulating air, in the combustion chamber, and in the heat exchanger. Since the air is no longer circulating through the structure being heated, this residual heat energy is lost up the stack and through case transfer into the immediate space, rather than being utilized to heat circulating air and the spaces being heated thereby. These inefficiencies reoccur each and every time the furnace cycles on and off.
Furthermore, many furnaces are oversized in order to ensure adequate capacity. They, therefore, tend to cycle more often for short periods of time. Contrary to the usual belief, short cycling aggravates and increases the energy losses and inefficiencies. Multiple short cycling of the furnace tends to cause oscillation of the temperature in the spaces being heated. This type of variation in the air temperature results in a lower comfort level than a steady state temperature. As a result, the thermostat is often set at a higher value than might be the case if a more constant temperature were maintained, requiring unnecessary utilization of fuel to maintain the higher thermostat set point temperature in the spaces being heated.
While hot water systems are somewhat different, they also incorporate operating heat losses and inefficiencies. Existing hot water heaters, particularly for small structures such as residences, also have high-stack and/or stand-by losses. In one type of hot water system in which a circulating water pump and the burner are energized and deenergized simultaneously, the residual heat in the water when the burner and pump are shut down is lost due to the lack of circulation and is transferred up the stack.
Thus, normally, the fan or circulator is energized when the temperature in the plenum reaches a selected value and is deenergized when the temperature sensed at that point reaches a second lower value. This hysteresis, i.e., the energizing of the fan or circulator at one temperature value and the deenergization of the fan or circulator at a different, lower value, is necessary in order to avoid frequent and rapid cycling of the fan or circulator as a function of small temperature differences.
In addition, in order to minimize the possibility of the fan or circulator being inadvertently energized independently of operation of the heating system itself, e.g., inadvertent operation at high ambient temperatures that might occur, for example, in the summer, the temperature at which the fan is turned on must be selected to be sufficiently high so that the fan will not be energized in response to hot ambient temperatures. As a result, the fan energization is delayed for a significant period of time after the heating system is energized, which precludes early efficient transfer of the heat energy being produced in the heating system to the fluid to be circulated and results, as described above, in undesirable losses.
Thermal hysteresis is provided because, otherwise, a fan might be deenergized shortly after the heating system was deenergized. As a result of the build-up of unused heat in the furnace thereafter, the fan might cycle on and off until the heat was dissipated from the heating system. For this reason, the temperature differential between the turn on and turn off values is usually at least 20.degree., and sometimes more. This allows the fan to continue to operate for a period of time after the heating system itself is deenergized so that it will not be restarted due to thermal conditions within the furnace system. The temperature at which the fan is turned on is normally set sufficiently high, typically in the neighborhood of 110.degree. or 120.degree., to preclude the fan from operating inadvertently during hot weather conditions.
Systems that operate under direct control of thermostats turn on when the thermostat initiates operation of the furnace and turn off when the thermostat opens a circuit to the furnace to deenergize the burner or heating system. No consideration in such systems is provided for extraction of maximum heat from the system at the end of the heating cycle.
Typical air conditioning (i.e., cooling) systems operate in this manner. In these systems, the air circulation and the compressor are energized and deenergized simultaneously. Upon deenergization, there is still a substantial amount of cooling capability left in the cooling system and in the circulating air, which could be extracted and used, thereby reducing the number of cycles of the cooling system.
Heating and cooling efficiencies could be improved if circulation of fluids, such as air in a forced air system, could be controlled to increase extraction of the thermal energy therefrom, whether for heating or cooling. One of the benefits from this approach should be to reduce the number of cycles and the inefficiencies resulting from, and occurring during, the initiation and ending of each cycle by extending the length of the cycle and extracting available energy from the circulating fluids, such as forced air.